As more and more nodes (e.g., routers) are added into a conventional communications network, the size of the network increases, and issues such as scalability and slow convergence may arise. In communication networks such as the Internet, an autonomous system (AS) may have a common routing policy (either in a single network or in a group of networks) that is controlled by a network administrator (or group of administrators on behalf of a single administrative entity, such as a university, a business enterprise, or a business division). Within the Internet, an AS comprises a collection of routers exchanging routing information via a common routing protocol. Each AS on the Internet may be assigned a globally unique number, which is sometimes called an AS number (ASN).
In a network comprising a single autonomous system (AS) with a single area, each node needs to be aware of the positional relationships (i.e., adjacencies) of all other nodes, such that all nodes may build a topological map of the AS. Nodes may learn about one another's adjacencies by flooding link-state information throughout the network according to one or more interior gateway protocols (IGPs) including, but not limited to, open shortest path first (OSPF) or intermediate system (IS) to IS (IS-IS). Specifically, nodes engaging in IGPs may distribute their own link state advertisements (LSAs) describing their own adjacencies to all their neighboring nodes, which may forward the received LSAs to all their neighboring nodes (except the node from which the LSA was received). This may allow the LSA to be distributed throughout the network such that all network nodes become aware of one another's adjacencies, thereby allowing the various nodes to build topology graphs (e.g., link state databases (LSDBs)). LSAs may be flooded upon network initialization as well as whenever a network adjacency changes (e.g., a node is added/removed or a node/link fails). A network change may lead to every node in the network to re-compute a shortest path to each destination, to update its routing information base (RIB) and its forwarding information base (FIB). Consequently, as more nodes are added to a network, link state distributions and shortest path computations may begin to consume more and more network resources, such as bandwidth and/or processing time.
A prior art technique for addressing scalability and performance issues in large networks is to define smaller areas of IGP (e.g., OSPF areas or IS-IS areas/levels) in an attempt to reduce the number of LSAs that are flooded throughout the network. This technique has been described by various publications, such as the Internet Engineering Task Force (IETF) publication request for comments (RFC) 2328 entitled “OSPF Version 2” (describing OSPF areas in an AS) and IETF publication RFC 1142 entitled “Open Systems Interconnection (OSI) IS-IS Intra-domain Routing Protocol” (describing IS-IS areas/levels in an AS). Specifically, each OSPF/IS-IS area comprises a number of interconnected routers, including both area border routers (ABRs) and internal routers. An ABR may be distinguished from an internal router in that the ABR may be connected to routers in two or more OSPF/IS-IS areas, while an internal router in an OSPF/IS-IS area may be connected only to other routers within the OSPF/IS-IS area. In most applications, the ABRs and internal routers will execute a normal link state distribution (e.g., according to an IGP) within their respective local OSPF/IS-IS areas, thereby allowing the ABRs to collect and summarize topology information (e.g., construct summary LSAs) describing their local OSPF/IS-IS area. Thereafter, the ABRs may distribute these summary LSAs to other ABRs on a backbone, thereby allowing the ABRs in external domains to develop a complete or partial topological understanding of the OSPF/IS-IS areas along the backbone. Depending on the network configuration, these summarized LSAs for an OSPF/IS-IS area may or may not be distributed to internal routers within the other OSPF/IS-IS areas.
Through splitting a network into multiple areas, the network may be further extended. However, there are a number of issues when splitting a network into multiple areas. For example, dividing an AS into multiple ASs or an area into multiple areas may involve significant network architecture changes. For another example, it may be complex to setup a multi-protocol label switching (MPLS) traffic engineering (TE) label switching path (LSP) crossing multiple areas. In general, a TE path crossing multiple areas may be computed by using collaborating path computation elements (PCEs) through the PCE communication protocol (PCEP), which may not be easy to configure by operators since manual configuration of the sequence of domains is required. Further, the current PCE method may not guarantee that the path found would be optimal. For yet another example, some policies may need to be reconfigured on ABRs for reducing the number of link states such as summary link-state advertisements (LSAs) to be distributed to other routers in other areas.
Furthermore, route convergence may be slower. A router in an OSPF area may see all other routers in the same area. A link-state change anywhere in an OSPF area may be populated everywhere in the same area, and may even be distributed to other areas in the same AS indirectly. For instance, all the routers and links in a point-of-presence (POP) in an OSPF area may be seen by all the other routers in the same area. Any link state change in the POP may be distributed to all the other routers in the same area, and may also be distributed to routers in other areas indirectly. A link state change in an area may lead to every router in the same area to re-calculate its OSPF routes, update its RIB and FIB. It may also lead to a number of link state distributions to other areas, which may trigger routers in other areas to re-calculate their OSPF routes, and update their RIBs and FIBs. Consequently, route convergence may become slower. As such, a simple and efficient scheme to address the above issues (e.g., in large networks) may be desired.